U.S. patent application number 12/901468 was filed with the patent office on 2011-04-14 for enhanced integrity projection lens assembly.
Invention is credited to Johan Joost Koning, Bart Schipper, Stijn Willem Herman Steenbrink.
Application Number | 20110084220 12/901468 |
Document ID | / |
Family ID | 43854096 |
Filed Date | 2011-04-14 |
United States Patent
Application |
20110084220 |
Kind Code |
A1 |
Koning; Johan Joost ; et
al. |
April 14, 2011 |
ENHANCED INTEGRITY PROJECTION LENS ASSEMBLY
Abstract
The present invention relates to a projection leis assembly
module for directing a multitude of charged particle beamlets onto
an image plane located in a downstream direction, and a method for
assembling such a projection lens assembly. In particular the
present invention discloses a modular projection lens assembly with
enhanced structural integrity and/or increased placement precision
of its most downstream electrode.
Inventors: |
Koning; Johan Joost; (Den
Haag, NL) ; Steenbrink; Stijn Willem Herman; (Den
Haag, NL) ; Schipper; Bart; (Nijkerk, NL) |
Family ID: |
43854096 |
Appl. No.: |
12/901468 |
Filed: |
October 8, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61250336 |
Oct 9, 2009 |
|
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Current U.S.
Class: |
250/492.3 ;
250/396R; 29/592.1 |
Current CPC
Class: |
B82Y 40/00 20130101;
H01J 2237/1205 20130101; B82Y 10/00 20130101; H01J 2237/121
20130101; H01J 3/26 20130101; H01J 37/3177 20130101; H01J 37/12
20130101; Y10T 29/49002 20150115; H01J 2237/04924 20130101 |
Class at
Publication: |
250/492.3 ;
250/396.R; 29/592.1 |
International
Class: |
G21K 1/08 20060101
G21K001/08; H05K 13/00 20060101 H05K013/00 |
Claims
1. Projection lens assembly for directing a multitude of charged
particle beamlets onto an image plane, said projection lens
assembly comprising a first electrode and a second electrode for
focusing one or more charged particle beamlets of the multitude of
charged particle beamlets, a housing comprising a through-opening
for allowing the multitude of charged particle beamlets to pass
through, said first and second electrodes each comprising a lens
hole-array aligned with the through-opening for allowing one or
more charged particle beamlets of the multitude of charged particle
beamlets to pass through, characterized in that said housing
comprises a circumferential wall and has an upstream and a
downstream distal edge, and the projection lens assembly further
comprises: at least one support element comprising a
through-opening for allowing the multitude of charged particle
beamlets to pass through, wherein said at least one support element
is attached to the housing, and wherein the first electrode and
second electrode are supported by the at least one support element,
wherein the first and second electrode are arranged in or near a
plane defined by the downstream distal edge of the housing.
2. Projection lens assembly according to claim 1, wherein the first
electrode and second electrode are attached to the support element
by means of an adhesive connection.
3. Projection lens assembly according to claim 1 or claim 2,
wherein the support element is constructed of a non-conductive
material.
4. Projection lens assembly according to any one of the preceding
claims, comprising a support element of the one or more support
elements at one or both of the downstream and upstream distal edge
of the housing.
5. Projection lens according to any one of the preceding claims,
wherein the second electrode forms a distal end of the projection
lens assembly in the downstream direction.
6. Projection lens assembly according to any one of the preceding
claims, wherein the first and second electrodes are arranged on an
upstream surface and a downstream surface of the support element
respectively, the first electrode being spaced apart from said
circumferential wall, and the support element bridging the distance
between the first electrode and the circumferential wall.
7. Projection lens assembly according to claim 6, wherein an
upstream surface of the support element is at least partially
covered with a first conductive coating connected to the first
electrode.
8. Projection lens assembly according to claim 6 or 7, wherein a
downstream surface of the support element is substantially covered
with a second conductive coating connected to the second
electrode.
9. Projection lens assembly according to claim 8, wherein the
second coating extends radially from the through-opening on the
downstream surface of the support element across an outer edge of
the support element to an outer peripheral area of the upstream
surface of the support element, and the first coating extends
radially outwards from the through-opening on the upstream surface
of the support element.
10. Projection lens assembly according to any one of the preceding
claims, further comprising a third electrode positioned upstream of
the first electrode.
11. Projection lens assembly according to claim 10, wherein the
third electrode and the second electrode are each kept at a
substantially constant potential, and wherein the potential of the
first electrode is adapted towards a predefined focus of charged
particle beamlets.
12. Projection lens assembly according to any one of the claims
5-11, wherein the circumferential wall is electrically conductive
and conductively connected, to the second coating.
13. Projection lens assembly according to any one of the preceding
claims, wherein the downstream facing and/or upstream facing
surfaces of the support element are fortified with strips of cured
adhesive material, preferably radially extending strips.
14. Projection lens assembly according to any one of the preceding
claims, wherein the support element further comprises a dielectric
breakdown protection structure at or near the periphery of the
through-opening.
15. Projection lens assembly according to claim 14, wherein the
dielectric breakdown protection structure has a stepped diameter
along the direction of the through-opening of the support
element.
16. Projection lens assembly according to any one of the preceding
claims, wherein the support element separates the first and second
electrode by less than 200 micron.
17. Projection lens assembly according to any one of the preceding
claims, wherein the support element, the first electrode and second
electrode are substantially planar.
18. Projection lens assembly according to any one of the preceding
claims, comprising a deflector unit arranged upstream of the
electrodes and adapted for providing a scanning deflection of the
multitude of charged particle beamlets.
19. Projection lens assembly according to any one of the preceding
claims, wherein the at least one support element comprises a first
support element, a second support element and a cover element, each
having a through-opening for the multitude of charged particle
beamlets, wherein said cover element is arranged at the upstream
distal edge of the circumferential wall, wherein the second support
element interconnects the cover element with the first electrode or
first support element.
20. Projection lens assembly according to claims 18 and 19, wherein
said second support element comprises the deflector unit.
21. Projection lens assembly according to claim 19 or claim 20,
wherein the cover element comprises a conductive material on an
upstream facing surface of the cover element and a non-conductive
material abutting the circumferential wall of the housing.
22. Projection lens assembly according to any one of the claims
18-21, further comprising a beam stop array arranged between the
deflector unit and the second electrode.
23. Projection lens assembly according to claim 22, wherein the
beam stop array and the most upstream electrode are spaced apart
and conductively connected by means of a conductive spacer.
24. Projection lens assembly according to claim 22 or claim 23,
wherein the deflector unit is adapted for deflecting a multitude of
charged particle beamlets around their associated pivot points
lying in substantially the same plane as the beam stop array.
25. Projection lens assembly according to any one of the claims
18-21, comprising a beam stop array arranged upstream of the
deflector unit, said deflector unit being adapted for deflecting a
multitude of charged particle beamlets around their associated
pivot points lying in a plane substantially between the first and
second electrode.
26. Charged particle beam lithography system for directing a
multitude of charged particle beamlets onto a target, said system
comprising a projection lens assembly according to any one of the
preceding claims.
27. Projection lens assembly according to any one of the preceding
claims, wherein the at least one support element comprises a cover
element comprising a through-opening for allowing the multitude of
charged particle beamlets to pass through, wherein the housing
comprises an upstream open end defined by the upstream edge,
wherein the cover element is adapted for substantially covering the
upstream open end of the housing wherein the first and second
electrodes are comprised in beam optics, and wherein the beam
optics are supported by the cover element by means of a body of an
adhesive applied on a downstream facing surface of the cover
element.
28. Projection lens assembly according to claim 27, wherein, at
least during construction thereof, the cover element is provided
with additional through-openings.
29. Projection lens assembly according to claim 27 or claim 28,
wherein the height of the body of adhesive is adapted to space the
cover element and the second electrode apart by a predetermined
distance.
30. Projection lens assembly according to any one of the claims
27-29, wherein the second electrode forms a distal end of the beam
optics in the downstream direction.
31. Projection lens assembly according to any one of the claims
27-30, further comprising one or more positioning elements fixedly
attached to said beam optics and circumferential wall in an
orientation substantially perpendicular to the direction of the
through opening, and adapted for positioning the beam optics at a
substantially fixed distance from the circumferential wall.
32. Projection lens assembly according to any one of the preceding
claims, wherein the circumferential wall comprises an electrically
conductive material.
33. Projection lens assembly according to claim 31 or claim 32,
wherein at least one of the positioning elements is electrically
conductive and conductively connects the second electrode with said
circumferential wall.
34. Projection lens assembly according to any one of the claims
27-33, wherein the cover element comprises a conductive surface on
an upstream facing surface of the cover element and a
non-conductive material abutting the circumferential wall of the
housing.
35. Projection lens assembly according to claim 34, wherein the
cover element comprises a conductive surface on a downstream facing
surface of the cover element, said conductive surfaces on the
upstream surface and downstream surface of the cover element
connecting and extending along the one or more through openings in
the cover element.
36. Projection lens assembly according to claim 35, wherein the
body of adhesive is a body of an electrically conductive adhesive
conductively connecting the beam optics and the conductive surface
on the downstream facing surface of the cover element.
37. Projection lens assembly according to any one of the claims
27-36, said cover element further comprising one or more
encapsulation rings comprising an electrically insulating material,
and arranged over a boundary of the conductive surface and the
non-conductive material of the cover element.
38. Projection lens assembly according to any one of the claims
27-37, wherein said beam optics further comprise a deflector unit
arranged upstream of the first electrode and adapted for providing
a scanning deflection of the multitude of beamlets.
39. Projection lens assembly according to claim 38, wherein the
beam optics further comprise a beam stop array arranged between the
deflector unit and the second electrode.
40. Projection lens assembly according to claim 39, further
comprising a conductive spacer, said spacer abutting and
conductively connecting the beam stop array and the first
electrode.
41. Projection lens assembly according to claim 38 or claim 39,
wherein the deflector unit is adapted for providing a scanning
deflection for a multitude of beamlets around their associated
pivot points lying in substantially the same plane as the beam stop
array.
42. Projection lens assembly according to claim 38, comprising a
beam stop array arranged upstream of the deflector unit, wherein
the deflector unit is adapted for deflecting a multitude of
beamlets around associated pivot points lying in a plane
substantially between the first and second electrodes.
43. Projection lens assembly according to any one of the claims
27-42, wherein the body of adhesive is applied between the cover
element and the next downstream structure to form a substantially
ring-shaped connection there between.
44. Projection lens assembly according to any one of the claims
27-43, wherein the adhesive body has a height between 2 millimeters
and 2 centimeters in the downstream direction.
45. Projection lens assembly according to any one of the claims
27-44, wherein the cover element substantially completely supports
the beam optics.
46. Projection lens assembly according to any one of the claims
27-45, wherein the cover element is one of the at least one or more
support elements.
47. Projection lens assembly according to any one of the preceding
claims, wherein the assembly is adapted to be placed and/or
replaced in a lithography system as a single unit.
48. Multiple charged particle beam lithography system comprising a
projection lens assembly according to any one of the claims
1-47.
49. Method for assembling a projection lens assembly for directing
a multitude of charged particle beamlets onto an image plane, said
projection lens assembly comprising: a housing comprising a
circumferential wall and having an upstream and a downstream distal
edge, beam optics comprising a first electrode and a second
electrode for focusing one or more charged particle beamlets of the
multitude of charged particle beamlets, and a cover element, said
housing and cover element comprising a through-opening for allowing
the multitude of charged particle beamlets to pass through, and
said first and second electrodes each comprising a lens-hole array
for allowing one or more charged particle beamlets of the multitude
of charged particle beamlets to pass through, wherein the cover
element is adapted for substantially covering the upstream edge of
the housing wherein said method comprises the steps of: aligning
the beam optics with the through opening of the cover element such
that the multitude of charged particle beamlets may pass through,
and such that the beam optics and the cover element are spaced
apart by a gap, fixing the cover element to the housing such that
it overlaps the upstream edge of the housing. filling the gap
between the beam optics and the cover element with a body of
adhesive for substantially supportingly bonding the beam optics to
the cover element, letting the body of adhesive cure.
50. Method according to claim 49, comprising an additional step of:
adjusting a distance between the beam optics and the cover element
while the adhesive has not yet cured.
51. Method according to claim 50, wherein the distance between the
beam optics and the cover element is adjusted such that a distance
between the second electrode and the cover element is equal to a
predetermined distance.
52. Method according to claim 51, comprising during the step of
adjusting said distance, a step of directing a multitude of
beamlets through the beam optics towards a beamlet profile sensor
arranged in a target plane, said beamlet profile sensor adapted for
measuring a corresponding multitude of beamlet profiles, and
varying said distance until an optimal measured focus of the
multitude of beamlets has been reached.
53. Method according to any one of the claims 49-52, wherein, at
least during assembly, the cover element comprises additional
through openings arranged around the through opening, wherein the
step of filling the gap between the beam optics and the cover
element comprises injecting the adhesive through said additional
through openings.
54. Method according to any one of the claims 49-53, wherein the
projection lens assembly further comprises positioning elements
adapted for positioning the beam optics at predetermined distances
from the circumferential wall, and wherein the method further
comprises a step of attaching the positioning elements to the beam
optics and the circumferential wall.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a projection lens assembly
for directing a multitude of charged particle beamlets onto an
image plane. In particular the present invention relates to a
robust and compact projection lens assembly allowing easy
handling.
BACKGROUND OF THE INVENTION
[0002] U.S. Pat. No. 6,946,662 discloses a lens assembly for
directing a multitude of charged particle beamlets onto an image
plane. The lens assembly comprises a plurality of electrodes having
a beam passing region where a plurality of charged beam apertures
are formed. The electrodes are stacked along an optical path with
insulator members spacing the electrodes apart. At the edge of the
electrodes, said electrodes are clamped together for forming a lens
assembly. The image provided by the lens assembly is reduced and
projected onto a wafer via a reduction electron optical system. The
lens assembly is used to correct beam aberrations which occur later
on in the path of projection when the beams are demagnified.
[0003] A disadvantage of the system disclosed in U.S. Pat. No.
6,946,662 is, that the lens assembly needs to be complex in order
to provide for the required corrections.
[0004] U.S. Pat. No. 7,091,504 to Applicant discloses an electron
beam exposure apparatus comprising a focusing corresponding
individual beamlet to a cross-section smaller than 300 nm, directly
onto the wafer. Since this system does not need a reduction
electron optical system, the effects of beam aberrations due to
this reduction electron optical system can be circumvented.
[0005] A disadvantage of the system disclosed in U.S. Pat. No.
7,091,504 is that the electron optical system needs to be arranged
much closer to the target.
[0006] Furthermore, in order to provide a stable electrostatic
field in the downstream direction, the electrode substrates of the
prior art, as for example disclosed in U.S. Pat. No. 6,946,662, are
thin and arranged close to each other, i.e. spaced apart by small
distances using insulator members at the sides of the electrodes.
Such thin electrodes form the weakest part of a projection lens
assembly; the electrodes may break or deform easily when handled,
and when sparks occur between the electrodes as a result of the
high potential difference there between, the electrodes are usually
damaged too severely to be of further use. Replacement of the
electrodes can result in significant downtime of a lithography
apparatus in which they are used.
[0007] It is a goal of the present invention to provide a
structurally robust compact modular projection lens assembly for
projecting a multitude of charged particle beamlets, e.g. tens of
thousands of beamlets or more, directly onto a target. It is a
further goal of the present invention to provide a modular
projection lens assembly which can be easily handled and maintained
and which may be placed as one unit.
[0008] It is a further goal of the present invention to provide a
compact modular projection lens assembly that is easily assembled
to within precise specifications, and a method for assembling such
a projection lens assembly.
SUMMARY OF THE INVENTION
[0009] To this end, according to a first aspect, the present
invention provides a projection lens assembly for directing a
multitude of charged particle beamlets onto an image plane, said
projection lens assembly comprising a first electrode and a second
electrode for focusing one or more charged particle beamlets of the
multitude of charged particle beamlets, a housing comprising a
through-opening for allowing the multitude of charged particle
beamlets to pass through, said first and second electrodes each
comprising a lens hole-array aligned with the through-opening for
allowing one or more charged particle beamlets of the multitude of
charged particle beamlets to pass through, wherein said housing
comprises a circumferential wall and has an upstream and a
downstream distal edge, and the projection lens assembly further
comprises: at least one support element comprising a
through-opening for allowing the multitude of charged particle
beamlets to pass through, wherein said at least one support element
is attached to the housing, and wherein the first electrode and
second electrode are supported by the at least one support element,
wherein the first and second electrode are arranged in or near a
plane defined by the downstream distal edge of the housing. During
handling of projection the lens assembly, the electrodes are
substantially protected by the circumferential wall of the housing.
Moreover, as the electrodes are located at a downstream part of the
housing, the projection lenses for the charged particle beamlets
may be placed close to the target.
[0010] In an embodiment the first electrode and second electrode
are attached to the support element by means of an adhesive
connection. The second electrode can thus be placed very close to
the target, i.e. at a distance in the range of 50 to 100 micron, as
no clamps or screws are used which project further downstream than
the second electrode itself. Moreover, during construction of the
projection lens assembly, the distance between the electrodes may
be easily adjusted by using more or less adhesive in the adhesive
connection and then letting the adhesive cure.
[0011] In an embodiment the support element is constructed of a
non-conductive material. The support element may thus be used as an
electrical insulator.
[0012] In an embodiment the projection lens assembly comprises a
support element of the one or more support elements at one or both
of the downstream and upstream distal edge of the housing. In a
preferred embodiment the projection lens assembly comprises a
support element at the downstream distal edge of the housing and
another support element at the upstream distal edge of the
housing.
[0013] In an embodiment the second electrode forms a distal end of
the projection lens assembly in the downstream direction. Thus the
second electrode may be placed very close to the target, to within
50 micron.
[0014] In an embodiment the first and second electrodes are
arranged on an upstream surface and a downstream surface of the
support element respectively, the first electrode being spaced
apart from said circumferential wall, and the support element
bridging the distance between the first electrode and the
circumferential wall. As the most downstream electrode of the
projection lens assembly is also the downstream distal end of the
projection lens assembly, the distance between the most downstream
electrode of the projection lens assembly and a target to be
patterned may be minimized. Moreover the support element, in
addition to supporting the electrodes, serves to disperse force
exerted on the electrodes and prevents deformation thereof.
Preferably the support element comprises a main layer of an
insulating material, such as borosilicate glass, which may be
easily machined to precise specifications and tolerances. As the
electrodes are thus mounted robustly in the projection lens
assembly and to some extent protected from shocks and other impact
forces, the present invention provides a robust module which may be
replaced relatively easily and in one piece.
[0015] In an embodiment an upstream surface of the support element
is at least partially covered with a first conductive coating
connected to the first electrode. Leads may easily be attached
anywhere on the first coating to provide the first electrode with a
potential for focusing the charged particle beamlets.
Alternatively, the leads may be directly connected to the first
electrode.
[0016] In an embodiment a downstream surface of the support element
is substantially covered with a second conductive coating connected
to the second electrode. The two coatings have a dual function.
Firstly, they act as a conductor for establishing a potential
difference between electrodes, and secondly the coatings
substantially shield the downstream end of the lens assembly from
external electromagnetic radiation and may thus form part of a
Faraday cage.
[0017] In an embodiment the projection lens assembly further
comprises a third electrode arranged upstream of the first
electrode. Preferably, the third electrode and the second electrode
are kept at constant potentials, whereas the potential of the first
electrode may be calibrated to fixate the strength of focus for
each individual projection lens assembly to a specified level. Thus
an array of lenses for adjustable focusing charged particle
beamlets is formed, for which an adjustment in focus does not
substantially influence electrical fields outside the volume
spanned by the third and second electrodes.
[0018] In an embodiment the second coating extends radially from
the through-opening at the downstream surface of the support
element across an outer edge of the support element to an outer
peripheral area of the upstream surface of the support element, and
the first coating extends radially outwards from the
through-opening at the upstream surface of the support element.
Preferably the second coating completely covers the downstream
facing surface and outer edge of the support element. Conductive
leads for the first electrode may be attached anywhere on the first
coating which is conductively connected therewith. This facilitates
placement of the leads for the electrodes
[0019] In an embodiment the downstream facing and/or upstream
facing surfaces of the support element are fortified with radially
extending strips of adhesive. The supporting capacity of the
support element can thus be increased, even after manufacture
thereof.
[0020] In an embodiment the circumferential wall is electrically
conductive and connected to the second coating. The circumferential
wall and the second coating thus provide substantial
electromagnetic shielding for the charged particle beamlets
traversing the projection lens assembly. Moreover, a support
element of this construction may be glued to the downstream distal
edge of the circumferential wall using a conductive adhesive
without requiring any further leads to be attached to the second
coating or second electrode.
[0021] In an embodiment the support element further comprises
dielectric breakdown protection structures at or near the periphery
of the through-opening. Preferably the through-opening of the
support element has a stepped diameter which is smaller near the
upstream surface of the support element and larger near the
downstream surface of the support element. Additionally there may
also be dielectric breakdown protection structures at or between
the outer peripheries of the first and/or second coatings. The
protection structures prevent sparking between the first and second
electrodes, allowing higher voltages to be applied between the
first and second electrodes and thus more uniform electric fields
for focusing the charged particle beamlets.
[0022] In an embodiment the non-conductive material, of the support
element, preferably comprised of borosilicate glass, separates the
first and second electrodes by a distance of about 200 microns or
less.
[0023] In an embodiment the support element as well as the first
electrode and/or second electrode are substantially planar.
[0024] In an embodiment the at least one support element comprises
a first support element, a second support element and a cover
element, each with a through-opening for the multitude of charged
particle beamlets, wherein said cover element is arranged at the
upstream distal, edge of the circumferential wall, the second
support element interconnecting the cover element with the first
electrode or first support element. During manufacture of the lens
assembly, once the circumferential wall and the support element
have been attached to each other, the cover element can be used to
substantially seal off the upstream side of the housing. Any space
between the second support element and the cover element can be
filled up with an adhesive. This design allows for some relaxation
of manufacturing tolerances while increasing the structural
integrity for the entire projection lens assembly.
[0025] In an embodiment the second support element is provided with
a deflector unit for providing a deflection of the multitude of
beamlets in a scanning direction. Advantageously, the deflector
unit and the focusing lens electrodes only have to be aligned
during construction of the projection lens assembly, not during
insertion of the projection lens assembly in a lithography system,
thus reducing the downtime of a lithography system when the
projection lens assembly is inspected or replaced.
[0026] In an embodiment the cover element comprises a conductive
material on an upstream facing surface of the cover element and a
non-conductive material abutting the circumferential wall of the
housing. The conductive upstream surface may be conductively
connected with the deflector unit. The cover element may thus
provide some shielding from external electromagnetic radiation
while being electrically isolated from the circumferential
wall.
[0027] In an embodiment the projection lens array further comprises
a beam stop array for at least partially stopping charged particle
beamlets that have been deflected by the beam blanker array from
reaching the target. When the beam stop array is arranged upstream
of the deflector unit, the pivot points of the charged particle
beamlets for scanning movement thereof may be arranged
substantially close to the image plane of the lens formed by the at
least two electrodes. Alternatively, the beam stop array may be
arranged between the lens and the deflector unit. The pivot points
of the beamlets for scanning movement thereof may then be arranged
in substantially the same plane as the beam stop array.
[0028] In an embodiment the deflector unit is adapted for
deflecting a multitude of beamlets around their associated pivot
points lying in substantially the same plane as the beam stop
array. As a result, any scanning deflection of the charged particle
beamlets by the deflector unit does not substantially change the
position of the beamlet spots on the beam stop array. The apertures
in the beam stop array can thus be kept small, in particular
smaller than the diameter of a single beamlet. Obviously, any
non-scanning deflection of a beamlet, for example by a beam blanker
placed upstream of the deflector unit, will move the beamlet spot
on the beam stop array to a position away from its associated
aperture, preventing the beamlet from traveling past the beam stop
array.
[0029] In an embodiment the at least one support element comprises
a cover element with a through-opening for allowing the multitude
of charged particle beamlets to pass through, wherein the housing
comprises an upstream open end defined by the upstream edge,
wherein the cover element is adapted for substantially covering the
upstream open end of the housing wherein the first and second
electrodes are comprised in beam optics, and wherein the beam
optics are supported by the cover element by means of a body of an
adhesive applied on a downstream facing surface of the cover
element. In this embodiment a downstream part of the beam optics
may be substantially free-hanging. Thus, when the beam optics are
substantially vertically oriented, as is typical in charged
particle beam lithography systems, there is no further need support
the weight of the beam optics on a downstream part of the
circumferential wall of the housing. The height of the projection
lens assembly may be adjusted during construction by varying the
distance between the cover element and the beam optics without
having to take into account a distance between a downstream support
element and the downstream distal edge of the housing.
[0030] In an embodiment the cover element, at least during
construction of the lens assembly, is provided with additional
through-openings for applying the adhesive to the downstream facing
surface of the cover element from a location upstream of said cover
element. Additionally, these through openings may further be used
to reach parts of the interior of the projection lens assembly once
the cover has been placed on the circumferential wall. For instance
it is possible to glue the cover element to the beam optics via
these through openings. Besides facilitating construction of a
projection lens assembly, the additional through openings also
shorten the path gas molecules have to travel when the gas is
evacuated out of the interior of the projection lens assembly.
[0031] In an embodiment the height of the body of adhesive in the
downstream direction is adapted to space the cover element and the
second electrode apart by a predetermined distance, preferably
within a range of 2 millimeters to 2 centimeters.
[0032] In an embodiment the second electrode forms a distal end of
the beam optics in the downstream direction.
[0033] In an embodiment the second electrode forms a distal end of
the projection lens assembly in the downstream direction. It is
thus possible to place a target to be exposed to charged particle
beamlets very close to the second electrode, providing a sharp
focusing of said beamlets.
[0034] In an embodiment the projection lens assembly further
comprises one or more positioning elements fixedly attached to said
beam optics and circumferential wall in an orientation
substantially perpendicular to the direction of the through
opening, and adapted for positioning the beam optics at a
substantially fixed distance from the circumferential wall. Though
these positioning elements are generally not adapted for supporting
the entire weight of the beam optics when the projection lens
assembly is in a substantially vertical position of use, horizontal
alignment and/or fixation of the beam optics is adequately effected
by such positioning elements, providing a more robust and more
accurately aligned projection lens assembly.
[0035] In an embodiment the circumferential wall comprises an
electrically conductive material, providing improved
electromagnetic shielding of the interior of the projection lens
assembly.
[0036] In an embodiment at least one of the positioning elements is
electrically conductive and conductively connects the second
electrode with said circumferential wall. The housing and the
second electrode may thus be kept at the same potential without the
use of additional wiring. Preferably the electrically conductive
positioning element or elements are attached to the beam optics and
to the circumferential wall using a conductive adhesive.
[0037] In an alternative embodiment the positioning elements
electrically connect another part of the beam optics, e.g. another
electrode, with the circumferential wall.
[0038] In an embodiment the cover element comprises a conductive
surface on an upstream facing surface of the cover element and a
non-conductive material abutting the circumferential wall of the
housing. The cover element thus provides some shielding from
external electromagnetic radiation while being electrically
isolated from the circumferential wall.
[0039] In an embodiment the downstream facing surface of the cover
element comprises a conductive surface, said conductive surfaces on
the upstream surface and downstream surface of the cover element
connecting and extending along the one or more through openings in
the cover element. The conductive surfaces further improve the
electromagnetic shielding and also reduce build up of static
electricity on the cover element.
[0040] In an embodiment the body of adhesive is a body of an
electrically conductive adhesive conductively connecting the beam
optics and the conductive surface on the downstream facing surface
of the cover element. Both the upstream and downstream facing
conductive surfaces of the cover element may thus be conductively
connected with at least part of the beam optics without requiring
an additional wire to be attached there between.
[0041] In an embodiment the cover element further comprises one or
more encapsulation rings comprising an electrically insulating
material, and arranged over a boundary between the conductive
surface and the non-conductive material of the cover element. These
encapsulation rings substantially reduce the chance of sparks
occurring within the projection lens assembly.
[0042] In an embodiment the beam optics further comprise a
deflector unit arranged upstream of the first electrode and adapted
for providing a scanning deflection of the multitude of beamlets.
Preferably the deflector unit comprises a conductive outer surface
which is conductively connected to the conductive surface of the
cover element to provide a high degree of electromagnetic shielding
for the beamlets passing through the beam optics.
[0043] In an embodiment the beam optics further comprise a beam
stop array arranged between the deflector unit and the second
electrode. By placing the beam stop array relatively close to the
second electrode, dispersion of the beamlets before they pass the
second electrode is reduced, i.e. the beamlet profiles remain
sharply defined.
[0044] In an embodiment the projection lens assembly further
comprises a conductive spacer, said spacer abutting and
conductively connecting the beam stop array and the first
electrode. In this embodiment, the first electrode and the beam
stop array are at the same potential. A voltage induced by the
charged particles incident on the beam stop array or passing
through said beam stop array may be measured by measuring the
potential of the first electrode. Moreover, in this embodiment,
acceleration of the charged particles between the beam stop array
and the first electrode is avoided, as they are at the same
potential.
[0045] In an embodiment the deflector unit is adapted for providing
a scanning deflection for a multitude of beamlets around their
associated pivot points lying in substantially the same plane as
the beam stop array. In this embodiment, any scanning deflection of
the charged particle beamlets does not substantially change the
position of the beamlet spots on the beam stop array. The apertures
in the beam stop array can thus be kept small, in particular
smaller than the diameter of a single beamlet. It will be clear
that any non-scanning deflection of a beamlet, for example by a
beam blanker placed upstream of the deflector unit, will move the
beamlet spot on the blanker array to a position away from its
associated aperture, preventing the beamlet from traveling past the
beam stop array.
[0046] In an embodiment the projection lens assembly comprises a
beam stop array arranged upstream of the deflector unit, and the
deflector unit is adapted for deflecting a multitude of beamlets
around associated pivot points lying in a plane substantially
between the first and second electrodes. The beamlets may thus have
their pivot points very close to a target plane while staying
focused independent of the scanning deflection.
[0047] In an embodiment the body of adhesive is applied between the
cover element and the next downstream structure to form a
substantially ring-shaped connection there between. In a preferred
embodiment this connection is air-tight.
[0048] In an embodiment the assembly is adapted to be placed and/or
replaced in a lithography system as a single unit.
[0049] According to a second aspect, the present invention provides
a charged particle beam lithography system for directing a
multitude of charged particle beamlets onto a target, said system
comprising a projection lens assembly as described herein. In
particular such a system comprises a beam source arranged upstream
of the projection lens assembly for providing a multitude of
charged particle beamlets, and a beam blanker for providing a
blanking deflection of selected beamlets of the multitude of
beamlets. More in particular such a system is adapted for operating
in a vacuum environment. During patterning of the target, the
target and the circumferential wall are preferably kept at the same
potential, for instance by conductively connecting both to
earth.
[0050] According to a third aspect, the present invention provides
a method for assembling a projection lens assembly for directing a
multitude of charged particle beamlets onto an image plane, said
projection lens assembly comprising:
[0051] a housing comprising a circumferential wall and having an
upstream and a downstream distal edge,
[0052] beam optics comprising a first electrode and a second
electrode for focusing one or more charged particle beamlets of the
multitude of charged particle beamlets, and
[0053] a cover element,
[0054] said housing and cover element comprising a through-opening
for allowing the multitude of charged particle beamlets to pass
through, and said first and second electrodes each comprising a
lens-hole array for allowing one or more charged particle beamlets
of the multitude of charged particle beamlets to pass through,
[0055] wherein the cover element is adapted for substantially
covering the upstream edge of the housing
[0056] wherein said method comprises the steps of: [0057] aligning
the beam optics with the through opening of the cover element such
that the multitude of charged particle beamlets may pass through,
and such that the beam optics and the cover element are spaced
apart by a gap, [0058] fixing the cover element to the housing such
that it overlaps the upstream edge of the housing. [0059] filling
the gap between the beam optics and the cover element with a body
of adhesive for substantially supportingly bonding the beam optics
to the cover element, [0060] letting the body of adhesive cure.
This method of assembling a projection lens assembly allows the
total height of the lens assembly to be easily adjusted without
keeping a multitude of spacers with different heights at hand.
Moreover, as the beam optics, which may also include a deflector
unit for providing a scanning deflection of the beamlets, are
substantially supportingly bonded to the cover element,
construction and maintenance of the assembly is simplified; only
the distance of the beam optics to the cover element has to be
aligned, and as the projection lens assembly may be substantially
open on the downstream side components inside the housing are
easily accessed. Finally, as no additional support elements are
required for supporting the beam optics on a downstream side, a
lighter and more compact projection lens assembly may be
manufactured.
[0061] In an embodiment the method comprises an additional step of
adjusting a distance between the beam optics and the cover element
while the adhesive has not yet cured. Such a distance may thus be
very accurately adjusted, in a much shorter time than when using
preformed solid spacers. When the cover element and the beam optics
are spaced further apart during construction of the projection lens
assembly, the height of the body of adhesive will increase and its
width will decrease, and more adhesive may be applied to compensate
for this decrease. Likewise, when the beam optics and the cover
element are brought closer together, the height of the body of
adhesive will decrease and its width will increase. In this case,
some of the adhesive may be removed. In contrast, when only a thin
sheet of adhesive is used such adjustment is not possible as such a
sheet would be pulled apart when the cover element and the beam
optics are moved away from each other. Preferably an adhesive is
used which, when cured, has a thermal expansion coefficient similar
to that of the surfaces it bonds to. The orientation of the cover
element relative to the beam optics thus remains substantially
constant, even when the projection lens assembly heats up or cools
down. When the adhesive is a very low shrinkage adhesive, strain on
the surfaces it bonds during the curing process is minimized.
[0062] In an embodiment the distance between the beam optics and
the cover element is adjusted such that a distance between the
second electrode and the cover element is equal to a predetermined
distance. Thus projection lens assemblies of substantially equal
heights may be produced, even when the height of individual beam
optics may vary to some extent.
[0063] In an embodiment the method comprises, during the step of
adjusting said distance, a step of directing a multitude of
beamlets through the beam optics towards a beamlet profile sensor
arranged in a target plane, said beamlet profile sensor being
adapted for measuring a corresponding multitude of beamlet
profiles, and varying said distance until an optimal measured focus
for the multitude of beamlets has been reached. As long as the
adhesive has not yet cured focusing and other properties of the
projection lens assembly can be measured and at least partially
adjusted by slightly varying the position of the beam optics
relative to the cover element. Preferably, during this step, the
intensity of the beamlets is less than the intensity of beamlets
that would typically be used during exposure of a target.
[0064] In an embodiment the cover element comprises, at least
during assembly, additional through openings arranged around the
through opening, and the step of filling the gap between the beam
optics and the cover element comprises injecting the adhesive
through said additional through openings. The body of adhesive can
thus easily be formed by applying adhesive on the cover element
from various angles, as an inner side of the body of adhesive may
be reached through the through opening for allowing a multitude of
beamlets to pass through, and an outer side of the body of adhesive
may be reached through the additional through openings. Conversely,
the through openings may also be used to remove excess adhesive as
long as it has not cured. After the body of adhesive has been
formed the additional through openings may be closed of, for
instance by filling the additional through openings with adhesive
as well. Alternatively, the additional through openings may, when
left open, be used to draw a vacuum within the projection lens
assembly, or to circulate cleaning gases or plasma through the
projection lens assembly.
[0065] In an embodiment the projection lens assembly further
comprises positioning elements adapted for positioning the beam
optics at predetermined distances from the circumferential wall,
and the method further comprises a step of attaching the
positioning elements to the beam optics and the circumferential
wall. During construction and curing of the adhesive, these
positioning elements help in spacing the beam optics a desired
distance apart from the circumferential wall. Moreover, once the
projection lens assembly has been constructed, the positioning
elements prevent a pendulum-like motion of the downstream end of
the beam optics.
[0066] According to a fourth aspect the present invention provides
a projection lens assembly for directing a multitude of charged
particle beamlets onto an image plane, said projection lens
assembly comprising a housing having a circumferential wall and
being provided with an upstream and a downstream distal edge, a
first electrode and a second electrode for focusing one or more of
the multitude of charged particle beamlets, and a support element
comprising a non-conductive material, said housing and support
element comprising a through-opening for allowing the multitude of
charged particle beams to pass through, and said first and second
electrodes each comprising a lens hole-array aligned with the
through-opening for allowing one or more charged particle beamlets
from the multitude of charged particle beamlets to pass through,
wherein the support element is attached to the downstream distal
edge of the housing, with the second electrode forming a distal end
of the projection lens assembly in the downstream direction, and
the first electrode being spaced apart from said circumferential
wall, the support element bridging the distance between the first
electrode and the circumferential wall.
[0067] In an embodiment the first electrode and second electrode
are supported by the support element and arranged on or near the
support element. Thus, both electrodes may be arranged downstream
of the support element.
[0068] In a preferred embodiment the first and second electrodes
are arranged on the upstream surface and the downstream surface of
the support element respectively. Thus the first electrode is
protected from damage by the housing and the support element.
[0069] The various aspects and features described and shown in the
specification can be applied, individually, wherever possible.
These individual aspects, in particular the aspects and features
described in the attached dependent claims, can be made subject of
divisional patent applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0070] The invention will be elucidated on the basis of an
exemplary embodiment shown in the attached drawings, in which:
[0071] FIG. 1A shows a schematic diagram of a prior art charged
particle exposure system,
[0072] FIGS. 1B and 1C, show a schematic diagram of an alternative
charged particle exposure system, and a detail thereof,
[0073] FIG. 2A shows a cross sectional view of an embodiment of the
projection lens assembly according to the invention,
[0074] FIG. 2B shows an enlargement of portion 260 of FIG. 2,
[0075] FIG. 2C shows a cross sectional view of an alternative
embodiment of the projection lens assembly,
[0076] FIG. 2D shows a cross sectional view of an further
alternative embodiment of the projection lens assembly,
[0077] FIG. 3 shows a schematic diagram of an alternative electrode
arrangement of a projection lens assembly according to the present
invention,
[0078] FIGS. 4A and 4B show a cross-sectional view along line A-A
of FIG. 2B and a top view of the projection lens assembly
respectively,
[0079] FIG. 5A shows a cross sectional schematic diagram of a
projection lens assembly according to the invention with its beam
stop arranged between the deflector unit and the electrodes,
[0080] FIG. 5B shows a cross sectional schematic diagram of a
projection lens assembly according to the invention with its beam
stop arranged upstream of the deflector unit,
[0081] FIG. 6 shows a schematic cross-sectional view of an
embodiment of a projection lens assembly according to a further
embodiment of the present invention.
[0082] FIG. 7 shows a schematic cross-sectional view of a further
embodiment of a projection lens assembly according to the present
invention.
[0083] FIGS. 8A, 8B and 8C show a schematic cross-sectional view, a
top view and a bottom view respectively of a further embodiment of
a projection lens assembly according to the present invention.
[0084] FIG. 9 shows a cross-sectional schematic diagram of a
portion of a further embodiment of a projection lens assembly
according to the invention.
[0085] FIG. 10 shows a cross-sectional schematic diagram of a
portion of a projection lens assembly according to the
invention.
[0086] FIG. 11 shows a cross-sectional schematic diagram of a
portion of a projection lens assembly according to the
invention.
[0087] FIGS. 12A and 12B show flow charts of embodiments of the
method according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0088] An optical column 1 as known from the prior art is shown in
FIG. 1A. A charged particle beam source 2 emits a charged particle
beam which traverses a double octopole 3 and collimator lens 4
before impinging on an aperture array 5. The aperture array then
splits the beam into a multitude of charged particle beamlets which
are condensed by condenser array 6. At beam blanker array 7
individual beamlets may be blanked, i.e. may be deflected such that
they encounter beam stop array 8 later on in their trajectories
instead of passing through apertures in beam stop array 8. The
beamlets that have not been blanked then pass through a deflector
unit 9 which is adapted to provide a scanning deflection of said
beamlets in the X- and Y-directions. The deflector unit is
typically a macro deflector comprising a conductive material
extending over its outer surface. At the end of their trajectories
the beamlets that have not been blanked pass through a lens array
10 adapted for focusing said beamlets onto a surface of a target
11. The beam stop array 8, deflector unit 9 and lens array 10
together comprise projection lens assembly 12 which provides
blocking of blanked beamlets, scanning deflection of the multitude
of beamlets, and demagnification of unblanked beamlets.
[0089] FIG. 1B shows an alternative optical column 31. A charged
particle beam source 32 emits a charged particle beam which
traverses a double octopole 33 and collimator lens 34 before
impinging on an aperture array 35. The aperture array then splits
the beam into a plurality of charged particle subbeams which are
condensed by condenser array 36. At beam blanker array 37 the
subbeams are split into a multitude of beamlets. The individual
beamlets may be blanked, i.e. may be deflected such that they
encounter beam stop array 38 later on in their trajectories instead
of passing through apertures in beam stop array 38. The beamlets
that have not been blanked then pass through a deflector unit 39
which is adapted to provide a scanning deflection of said beamlets
in the X- and/or Y-direction. The deflector unit typically
comprises micro-electro mechanical system (MEMS) elements adapted
for provided scanning deflection of groups of said beamlets. At the
end of their trajectories the beamlets that have not been blanked
pass through a lens array 40 adapted for focusing said beamlets
onto a surface of a target 41. The beam stop array 38, deflector
unit 39 and lens array 40 together comprise projection lens
assembly 42 which provides blocking of blanked beamlets, scanning
deflection of the multitude of beamlets, and demagnification of
unblanked beamlets.
[0090] FIG. 1C schematically shows a detail of an optical column as
shown in FIG. 1B, showing trajectories of three beamlets b1, b2,
b3. A subbeam of a beam from a charged particle beam source emerges
from aperture array 35 and is condensed by condenser array 36. The
subbeam is subsequently split up into beamlets b1, b2, b3 at beam
blanker array 37, which is adapted for providing a blanking
deflection to the individual beamlets. In the figure shown, none of
the beamlets b1, b2, b3 is provided with a blanking deflection, so
that said beamlets pass through a common aperture in beam stop
array 38. The unblanked beamlets are then provided with a scanning
deflection by deflector unit 39, which comprises MEMS elements 39a
and 39b adapted for providing a scanning deflection to the
plurality of beamlets. At the end of their trajectories the
beamlets that have not been blanked pass through a lens array 40
adapted for focusing said beamlets onto a surface of a target 41.
The beam stop array 38, deflector unit 39 and lens array 40
together comprise a projection lens assembly which provides
blocking of blanked beamlets, scanning deflection of the multitude
of beamlets, and demagnification of unblanked beamlets.
[0091] FIG. 2A shows a cross sectional view of an embodiment of an
improved projection lens assembly 200 according to the invention.
The embodiment shown comprises a housing having an electrically
conductive circumferential wall 230, preferably metallic. The
projection lens assembly further comprises a cover element 210, and
a support element 240 at the downstream end of said housing. A
through-opening 213 for the passage of charged particle beamlets
extends from the upstream surface of the cover element 210, through
the interior of the projection lens assembly towards the first
electrode 201, through the support element 240 and finally
debouches in second electrode 202. A multitude of charged particle
beamlets may traverse said through-opening before impinging on a
target 270. In the embodiment shown, the support element extends
parallel to both the first and the second electrode. Preferably,
the support element extends radially away from the lens-hole arrays
in first and second electrodes.
[0092] To avoid formation of electrical fields between the target
and the projection lens assembly, both may be connected to ground
and/or conductively connected to each other. A structurally robust
projection lens assembly according to the invention may be placed
integrally in a known lithography system or may be swapped out or
removed for maintenance purposes.
[0093] The multitude of charged particle beamlets first passes
through the through passage 213 in the cover element 210. The main
body of the cover element 210 is made from a non-conductive
material, though its upstream surface comprises a conductive
coating 211, and a portion of its downstream surface comprises
another conductive coating 212, which both serve to shield the
inside of the lens assembly from external electromagnetic
influences. Preferably the two conductive coatings are joined at
the sides of the through opening, forming a single contiguous
surface. Encapsulation rings 251 and 252, made from a
non-conductive, preferably ceramic material, encapsulate corners
between the coatings and cover element 210, in order to reduce the
chance of sparks forming at those corners.
[0094] Once the charged particle beamlets have traversed the
through opening they pass through a deflector unit 220 which is
adapted to provide scanning deflection of a multitude of beamlets.
The deflector unit may deflect the charged particle beamlets any
number of times using a corresponding number of deflectors. In a
preferred embodiment the deflector unit comprises an X- and a
Y-deflector. Leads 224a and 224b, preferably triaxial leads, at
base 222 of the deflector unit are adapted for carrying control
signals indicating whether the deflector unit is functioning
properly. The deflector unit is substantially supported by the
support element 240, yet preferably also attached through a
conductive adhesive connection 221 to cover element 210. The
deflector unit thus acts a second support element which
interconnects the cover element with support element 240, thereby
increasing the structural robustness of the assembly.
Advantageously, a conductive adhesive is applied to facilitate
construction of the projection lens assembly, i.e. to allow for
widening of tolerances, thereby reducing costs and effort in
manufacture of elements on the one hand, as well as of the assembly
thereof on the other hand. According to insight underlying the
present invention in this respect, slight variations in distance
between the downstream facing conductive coating 212 of cover
element 210 and the deflector unit 220 are compensated for by
filling the gap with more or a lesser amount of conductive
adhesive. In a preferred embodiment, the adhesive is of a type
which, once cured, has a very low volatility in a vacuum and a
thermal expansion coefficient corresponding, if not similar to the
surfaces it bonds. The base 222 of the deflector unit is mounted on
an insulator 223 which electrically insulates the deflector unit
from first electrode 201. Both the first and second electrodes
201,202 comprise a lens hole-array, each lens hole corresponding to
a charged particle beamlet that might pass through it.
[0095] The support element 240 comprises a layer 243 of a
non-conducting material, a first conductive coating 241 on an
upstream facing surface of the support element, and a second
conductive coating 242 on an downstream facing surface of the
support element. The first and second conductive coatings are
electrically isolated from each other. The first electrode 201 is
conductively connected to the first conductive coating 241 on the
upstream facing surface of the support element. Leads 209a and
209b, which are attached to the first coating, are adapted for
providing an electrical signal to the first electrode, preferably
such that a potential difference between the first and second
electrodes which generates a field for focusing charged particle
beamlets, wherein the potential difference between said electrodes
is in the range of 4 kV. Second electrode 202, which is arranged on
the downstream facing surface of support element 240, is
conductively connected to said second conductive coating 242 which
preferably covers all or most of said downstream facing surface. In
a preferred embodiment, the second coating extends across the outer
edge of the support element and is in conductive contact with the
circumferential wall 230 of the housing. The circumferential wall
and first coating are thus adapted to provide at least partial
electromagnetic shielding of the interior of the lens assembly.
[0096] Advantageously, the insulating layer 243 preferably
comprising borosilicate glass, of the support element is strong
enough to substantially support the weight of the deflector unit,
yet thin enough to allow strong and uniform electrical fields to be
generated between the first and second electrodes.
[0097] FIG. 2B shows a magnified view of portion 260 of FIG. 2A. It
can clearly be seen that the first and second coatings 241,242
which have been deposited on insulating layer 243, are electrically
isolated from each other by the insulating layer and by gap 244a.
The gap 244a is filled with a non-conductive adhesive. Preferably
the adhesives used in the projection lens assembly have low thermal
expansion coefficients. To prevent sparks from occurring at corners
of the first and second coatings, dielectric breakdown protection
structure 253 has been placed at point of contact between the
circumferential wall 230 and the support element 240. A small
recess 246 in the insulating layer 243 at the through-opening of
the support element 240 serves to prevent sparks from occurring
between the first and second electrodes and their corresponding
coatings near the through-opening.
[0098] FIG. 2C shows an alternative embodiment of a projection lens
in which the weight of the deflector unit 220 is substantially
completely supported by the support element 240, i.e. the deflector
unit is not supportingly connected to the cover element 210.
Isolated electrical lead 225, preferably attached to the deflector
unit by means of a detachable connector, conductively connects the
outer surface of the deflector unit to the downstream facing
conductive coating of the cover element, ensuring that these have
substantially the same potential. In this embodiment, the cover
element is removably attached to the circumferential wall 230. In
another embodiment lead 225 passes through the circumferential wall
to connect with the upstream facing surface of the cover
element.
[0099] FIG. 2D shows an alternative embodiment of a projection lens
assembly according to the invention. The second electrode 202 and
the third electrode 203 are electrically connected to the housing
and substantially at ground potential. The first electrode 201 is
electrically insulated from the second electrode 202 by support
element 240, and electrically insulated from the third electrode
203 by insulator 223. Leads 209a and 209h are attached to the first
electrode 201 for providing an electrical signal to first electrode
such that an electrical field is generated between the first and
second electrode and between the first and third electrode, for
focusing charged particle beamlets. Typically, the potential
difference between said the first electrode 201 and the second and
third electrodes 202,203 in this embodiment is in the range of -3,4
kV. The outer edges of first electrode 201 are encapsulated by
bodies of a non-conductive adhesive, for preventing the forming of
sparks at the outer edges of the first electrode 201.
[0100] FIG. 3 shows a schematic diagram of a section of an
alternative projection lens assembly according to the invention.
The projection lens assembly is constructed in a manner similar to
the projection lens assembly shown in FIG. 2, and comprises,
besides first electrode 301 and a second electrode 302, a third
electrode 303 arranged upstream of the first electrode 301. Support
element 340 has been reinforced with strips of glue 345 which are
deposited on first conductive coating 341. Preferably, neighboring
elements are bonded to each other using suitable adhesives.
[0101] The third and first electrodes 303,301 are spaced apart and
electrically isolated from each other by insulating spacer 324. The
third electrode is conductively connected to the outer surface of
the deflector unit 320, through conductive beam stop array 322 and
conductive spacer 323. The outer surface of the deflector unit 320,
and therefore the third electrode 303 as well, is preferably kept
at a constant potential, for example at -4 kV with respect to
ground. The second electrode 302 is again conductively connected to
the circumferential wall (not shown) through second conductive
coating 342. Preferably the circumferential wall and second
electrode 302 are kept at the same substantially constant potential
as the target to be patterned, typically at ground potential. By
varying the potential of the first electrode 301, the electrical
fields between the first electrode and the third electrode 303 can
be varied, as well as the electrical field between the first
electrode 301 and the second electrode 302. The potential of the
first electrode may typically be varied in the range of -4.3 kV. In
this manner an array of electrostatic lenses is formed, capable of
generating adaptable electrical fields for focusing a multitude of
charged particle beamlets.
[0102] In an alternative embodiment, the third electrode and the
second electrode are kept substantially at ground potential, and
the first electrode is kept at a substantially constant potential,
for example at -3,4 kV with respect to the third electrode and the
second electrode. In this embodiment the insulating spacer 324 with
electrically insulates the first electrode and third electrode from
each other is preferably formed by applying an electrically
insulating adhesive between the first electrode and the third
electrode.
[0103] FIG. 4A shows a cross-sectional view of the projection lens
assembly of FIG. 2B along the line A-A. The support element 240,
which extends radially away from the lens-hole arrays of first
electrode 201 and second electrode 202 (not shown), comprises a
first conductive coating 241 on its upstream surface.
Circumferential wall 230 and dielectric protection structure 253
are shown in greater detail as well. The upstream surface of the
support element 240 is provided with radially extending strips 245
of adhesive which further increase the supporting capacity of the
support element. In case strips of, preferably non-conductive,
adhesive are provided on the downstream facing surface of the
support element, care must be taken that the ridges of the strips
do not project past the second electrode in the downstream
direction. The second electrode may then still be placed very close
to the target.
[0104] FIG. 4B shows a top view of the projection lens assembly of
FIG. 2A, in which the through-opening 213 for the multitude of
beamlets, dielectric breakdown protection ring 251 and upstream
facing conductive coating 211 of the cover element 210 are visible.
The ring 251, preferably comprising a non-conductive ceramic
material, encapsulates the outer edge of the upstream facing
conductive coating 211 to prevent sparks from forming at that
edge.
[0105] A schematic diagram of charged particle beamlet trajectories
in an embodiment of the projection lens assembly according to the
invention is given in FIG. 5A. In this embodiment a beam stop array
522 has been arranged between deflector unit 520 and third
electrode 503. Beamlets 510a, 510b and 510c pass through deflector
unit 520 which is adapted to provide a scanning deflection of said
beamlets in the X- and/or Y-directions. Enveloped by the conductive
outer surface of the deflector unit 520, are deflector plates 527
which are electrically isolated from the outer surface of the
deflector unit by spacers comprising an isolating material 526.
[0106] Before reaching the deflector unit, beamlet 510a has been
given a blanking deflection, and is therefore directed onto a
non-beamlet passing region of beam stop array 522. The unblanked
beamlets 510b and 510c traverse the deflector unit, and are
deflected about their corresponding pivot points Pb and Pc which
are located in substantially the same plane as beam stop array 522.
As the pivot point of each beamlet is in substantially the same
plane as the beam stop array, unblanked beam spots do not move on
the beam stop array, i.e. the deflection by the deflector unit does
not substantially influence the intensity distribution of unblanked
beamlets at their corresponding pivot points. Unblanked beamlets
510b and 510c then pass the lens holes in the first and second
electrodes 501, 502, between which electric fields 505a are
generated which are adapted to focus the charged particle beamlets
onto a target 570. The electrodes are spaced apart by spacers 523,
524 and 525. Because the beam stop array and the pivot points of
the beamlets in this arrangement may be positioned relatively close
to the lens electrodes, in particular closer to the principal plane
of the lens electrodes than if the beam stop array were placed
upstream of the deflector unit, beamlet aberration is significantly
reduced. This results in more sharply defined beamlet spots on the
target and allows patterning of the target at higher
resolutions.
[0107] FIG. 5B shows an alternative embodiment of a projection lens
assembly according to the invention, wherein the beam stop array
522 is arranged upstream of the deflector unit 520. Charged
particle beamlet 510a has been given a blanking deflection before
reaching the be stop array and therefore impinges on a non-beamlet
passing region thereof. Charged particle beamlets 510b and 510c
pass the beam stop array 522 and are given a scanning deflection by
deflector unit 520. The beamlets are then focused using electrodes
501, 502, 503 before striking the target 570. Because the second
electrode 502 forms the downstream distal end of the projection
lens assembly, the charged particle beamlets may have their pivot
points positioned very close to the target 570 while staying
focused substantially independent of the scanning deflection.
[0108] FIG. 6 shows an embodiment of an improved projection lens
assembly 600 according to the invention. The embodiment shown
comprises a housing having an electrically conductive
circumferential wall 230, preferably metallic. The projection lens
assembly further comprises a cover element 210 substantially
covering an opening at the upstream end of said housing. A
through-opening 213 for the passage of charged particle beamlets
extends from the upstream surface of the cover element 210, through
the interior of the projection lens assembly towards the first
electrode 201, through non-conductive spacer 215, and finally
debouches in second electrode 202. A multitude of charged particle
beamlets may traverse said through opening before impinging on a
target 270.
[0109] To avoid formation of electrical fields between the target
and the projection lens assembly, both may be connected to ground
and/or conductively connected to each other. A projection lens
assembly according to the invention may be placed integrally in a
known lithography system or may be swapped out or removed for
maintenance purposes.
[0110] The multitude of charged particle beamlets first passes
through the through passage 213 in the cover element 210. The main
body of the cover element 210 is made from a non-conductive
material, though its upstream surface comprises a conductive
surface 211, and a portion of its downstream surface comprises
another conductive surface 212, which both serve to shield the
inside of the lens assembly from external electromagnetic
influences. Preferably the two conductive surfaces join at the
sides of the through opening, forming a single contiguous surface.
Encapsulation rings 251 and 252, made from a non-conductive,
preferably ceramic material, encapsulate corners between the
conductive surfaces and cover element 210, in order to reduce the
chance of sparks forming at those corners.
[0111] Once the charged particle beamlets have traversed the
through-opening of the cover element, they pass through beam optics
217 which directs the multitude of beamlets onto target 270. In the
embodiment shown the beam optics comprise a scanning deflector unit
220 mounted at its base 222 on a non-conductive spacer 223, and
further comprise first electrode 201, non-conductive spacer 215 and
second electrode 202. Deflector unit 220 is adapted for providing a
scanning deflection of the multitude of beamlets. The deflector
unit may deflect the charged particle beamlets any number of times
using a corresponding number of deflectors. In a preferred
embodiment the deflector unit comprises an X- and a Y-deflector.
Leads 224a and 224b, preferably triaxial leads, at base 222 of the
deflector unit are adapted for carrying control signals indicating
whether the deflector unit is functioning properly. The beam optics
are substantially supported by the cover element 210 and attached
thereto through a conductive adhesive connection 221. According to
insight underlying the present invention in this respect,
variations in distance between the downstream facing conductive
surface 212 of cover element 210 and the deflector unit 220 may be
compensated for during construction of the projection lens
assembly, by adjusting the height of the body of conductive
adhesive 221 which fills the gap. In a preferred embodiment, the
adhesive is of a type which, once cured, has a very low volatility
in a vacuum and a thermal expansion coefficient corresponding, if
not similar to the surfaces it bonds. The base 222 of the deflector
unit is mounted on an insulator 223 which electrically insulates
the deflector unit from first electrode 201. Both the first and
second electrodes 201,202 comprise a lens-hole array, each hole of
said array corresponding to a charged particle beamlet that might
pass through it.
[0112] Lead 209, is attached to the first electrode 201 and adapted
for providing an electrical signal thereto, preferably such that a
potential difference between the first and second electrodes
201,202 which generates a field for focusing charged particle
beamlets, is in the range of 4 kV. Second electrode 202, which is
arranged on the downstream facing surface of positioning element
240, is conductively connected to the circumferential wall 230 by
means of conductive wire 218.
[0113] Typically the height of the beam optics, in this case the
distance from the downstream distal end of the second lens array
202 to the upstream distal end of the deflector unit 220, can not
be adjusted once the beam optics have been assembled. According to
the method of the present invention, the total distance from the
distal edge of the second electrode to the upstream surface of the
cover element may however quite easily be adjusted by varying the
height of the body of adhesive 221 which, bonds the two together.
Additional through openings 214a, 214b in the cover element 210 may
be used during construction of the projection lens assembly to
inject adhesive through to fill a gap between the beam optics and
the cover element.
[0114] FIG. 7 shows an alternative embodiment of a projection lens
assembly 700 according to the invention. In this embodiment the
beam optics 717 do not comprise a deflector unit for providing a
scanning deflection of the multitude of charged particle beamlets.
A deflector unit may be placed upstream of the projection lens
assembly in a lithography system. The beam optics 717 comprise a
first electrode 201 and a second electrode 202, spaced apart by a
non-conductive spacer 215 which also forms part of the beam optics.
The first electrode 201 is conductively connected to a downstream
facing surface of the cover element 210. The connection between the
downstream facing surface, which comprises conductive surface 212,
and the first electrode, is formed by a body of conductive adhesive
221. Lead 218 electrically connects the second electrode 202 with
the circumferential wall 230, such that both are a ground
potential. Especially when the electrodes are thin and placed close
to each other to provide a strong and uniform electric field, the
height of the body of adhesive is an important factor in the height
of the entire projection lens assembly. During construction of the
projection lens assembly, this height may conveniently be adjusted
to a desired height by varying the height of the body of
adhesive.
[0115] FIG. 8A shows an alternative embodiment of a projection lens
assembly similar to that shown in FIG. 6, further comprising
positioning elements 249a, 249b. Near the downstream opening of the
housing the beam optics are aligned substantially perpendicular to
the direction of the through opening by means of these positioning
elements 249a, 249b. Besides providing additional stability in the
position of the beam optics relative to the housing, the
positioning elements may also facilitate construction of the
projection lens assembly, i.e. simplify alignment of the beam
optics perpendicular to the direction of the through opening. In
this embodiment the positioning elements comprise elongated, thin
and substantially rigid structures, and are preferably attached to
the circumferential wall 230 and to the beam optics 217 using an
electrically conductive adhesive. In the embodiment shown, the
positioning elements comprise an electrically conductive material
and are attached to the second electrode 202 and to the
circumferential wall 230 thus keeping these at the same potential.
The positioning elements 249a, 249b keep the distance between the
otherwise free hanging distal end of the beam optics and the
circumferential wall substantially constant, increasing the
structural integrity of the projection lens assembly and
restraining translational and/or oscillating movement of the second
electrode 202 relative to the circumferential wall. In an
alternative embodiment, the positioning elements are used only
during construction of the projection lens assembly, and are not
present in the finished product.
[0116] FIG. 8B shows a top view of the projection lens assembly of
FIG. 6 along a section indicated by line A, i.e. not showing the
outermost peripheral area of cover element 210. Moving in a
direction radially outward from through opening 213 of said cover
element 210, an upstream facing conductive surface 211 can be seen
surrounding additional through openings 211a, 214b. The additional
through openings are arranged around through opening 213 and
facilitate passage of an injection needle or the like for
depositing a body of adhesive between the downstream facing surface
of the cover element and an upstream facing surface of the beam
optics. Moving further outward, an edge between upstream facing
conductive surface 211 and a non-conductive portion of cover
element 210 is encapsulated by encapsulation ring 251 comprised of
a non-conductive material, to reduce the chance of sparks occurring
at that edge.
[0117] FIG. 8C shows a bottom view of the projection lens assembly
of FIG. 6, along the section indicated by line A. Second electrode
202 is electrically connected to circumferential wall 230 by
positioning elements 249a,249b. Behind second electrode 202, part
of non-conductive spacer 215 can be seen, as can parts of the
additional through openings 214a,214b. These additional through
openings debouch in a conductive surface 212 of the cover element
210, said material enhancing the electromagnetic shielding
properties of the projection lens assembly. An edge between
conductive surface 212 and a non-conductive portion of the cover
element 210 is encapsulated by encapsulation ring 252, to prevent
sparks from occurring at that edge. Positioning elements 249a,249b
extend substantially perpendicular to the direction of the through
opening 213, and are adapted to substantially fix the position of
the beam optics along the plane which they span. In other words,
when the second electrode 202 and the positioning elements
249a,249b are oriented horizontally, the positioning elements
substantially constrain horizontal movement of the beam optics
relative to the circumferential wall. As the positioning elements
limit movement along the direction in which they extend, small
adjustments in distance between the beam optics and the cover
element can be made even when the positioning elements have already
been fixedly attached to the beam optics and to the circumferential
wall. In an alternative embodiment, the positioning element may be
formed by a membrane which is fixedly attached to the beam optics
and to the circumferential wall in an orientation substantially
perpendicular to the direction of the through opening, and adapted
for positioning the beam optics at a substantially fixed distance
from the circumferential wall.
[0118] FIG. 9 shows a schematic diagram of a section of an
alternative projection lens assembly according to the invention.
The projection lens assembly is constructed in a manner similar to
the projection lens assembly shown in figure BA, and comprises,
besides first electrode 301 and second electrode 302, a third
electrode 303, also provided with an aperture array, is arranged
upstream of the first electrode 301. Preferably, neighboring
elements are bonded to each other using suitable adhesives.
[0119] The third and first electrodes 303,301 are spaced apart and
electrically isolated from each other by insulating spacer 324. The
third electrode is conductively connected to the outer surface of
deflector unit 320, through conductive beam stop array 322 and
conductive spacer 323. The outer surface of deflector unit 320, and
therefore the third electrode as well, is preferably kept at a
constant potential, for example at -4 kV with respect to ground.
The second electrode 302 is electrically connected to the
circumferential wall (not shown) via positioning elements 349a and
349b, and insulated and spaced apart from first electrode 301 by
non-conductive spacer 315. The positioning elements serve to align
the downstream end of the beam optics along a plane spanned by the
second electrode, i.e. in the orientation shown the positioning
elements horizontally align the downstream end of the beam optics.
Non-conductive spacer 315 is provides with a stepped portion 346
which increases the length of a path from the first electrode to
the second electrode along the surface of spacer 315. This
increased path length helps to reduce the chance of spark occurring
between the first and second electrodes.
[0120] The circumferential wall and second electrode 302 are
preferably kept at the same substantially constant potential as the
target to be patterned, typically at ground potential. By varying
the potential of the first electrode 301, the electrical fields
between the first electrode 301 and the third electrode 303 can be
varied, as well as the electrical fields between the first
electrode 301 and the second electrode 302. The potential of the
first electrode may typically be varied in the range of -4.3 kV. In
this manner an array of electrostatic lenses is formed, capable of
generating adaptable electrical fields for focusing a multitude of
charged particle beamlets.
[0121] A schematic diagram of charged particle beamlet trajectories
in an embodiment of the projection lens assembly 700 according to
the invention is given in FIG. 10. In this embodiment a beam stop
array 522 has been arranged between deflector unit 520 and third
electrode 503. Beamlets 510a, 510b and 510c pass through deflector
unit 520 which is adapted to provide a scanning deflection of said
beamlets in the X- and Y-directions. Enveloped by the conductive
outer surface of the deflector unit 520, are deflector plates 527
which are electrically isolated from the outer surface of the
deflector unit by spacers comprising an isolating material 526.
[0122] Before reaching the deflector unit, beamlet 510a has been
given a blanking deflection, and is therefore directed onto a
non-beamlet passing region of beam stop array 522. The unblanked
beamlets 510b and 510c traverse the deflector unit, and are
deflected about their corresponding pivot points Pb and Pc which
are located in substantially the same plane as beam stop array 522.
As the pivot point of each beamlet is in substantially the same
plane as the beam stop array, unblanked beam spots do not move on
the beam stop array, i.e. the deflection by the deflector unit does
not substantially influence the intensity distribution of unblanked
beamlets at their corresponding pivot points. Unblanked beamlets
510b and 510c then pass the lens holes in the first and second
electrodes 501, 502, between which electric fields 505a are
generated which are adapted to focus the charged particle beamlets
onto a target 570. The electrodes are spaced apart by spacers 523,
524 and 515. Because the beam stop array and the pivot points of
the beamlets in this arrangement may be positioned relatively close
to the electrodes, in particular closer to the planes spanned by
the electrodes than if the beam stop array were placed upstream of
the deflector unit, beamlet aberration is significantly reduced.
This results in more sharply defined beamlet spots on the target
and allows patterning of the target at higher resolutions.
[0123] FIG. 11 shows an alternative embodiment of a projection lens
assembly according to the invention, wherein the beam stop array
522 is arranged upstream of the deflector unit 520. Charged
particle beamlet 510a has been given a blanking deflection before
reaching the beam stop array and therefore impinges on a
non-beamlet passing region thereof. Charged particle beamlets 510b
and 510c pass the beam stop array 522 and are given a scanning
deflection by deflector unit 520. The beamlets are then focused
using electrodes 501, 502, 503 before striking the target 570.
Because the second electrode 502 forms the downstream distal end of
the projection lens assembly, the charged particle beamlets may
have their pivot points positioned very close to the target 570
while staying focused substantially independent of the scanning
deflection. Though in the embodiment shown scanning deflection of
the beamlets is performed using a macro deflector which is adapted
for providing two electrical fields, in an alternative embodiment
the deflector unit may be adapted to provide several fields for
scanning deflection of the beamlets, e.g. one or more electrical
fields per beamlet, as shown, in FIG. 1A, or one or more electrical
fields per group of beamlets.
[0124] FIG. 12A shows a flow chart of a method for assembling some
of the embodiments mentioned above. In step 901 the beam optics are
aligned with the through opening of the cover element such that the
beam optics and the cover element are at a predetermined distance
from each other. Next, in step 902 the aligned cover element is
attached to an upstream end of the housing such that it overlaps
the upstream edge of the housing. In step 903 a gap between the
beam optics and the cover element is filled up with a body of
adhesive for substantially supportingly bonding the beam optics to
the cover element. Finally, in step 904 the body of adhesive is
allowed to cure. The method allows more convenient manufacture of
such projection lens assemblies, which can also be more accurately
dimensioned using the method.
[0125] FIG. 12B shows an alternative embodiment of the method, in
which the distance between the cover element and the downstream
distal edge of the beam optics of a projection lens assembly is
adjusted during construction of the projection lens assembly in
step 903b, before the adhesive cures. This method allows adjustment
of said distance to a predetermined value. During this adjustment
step it is possible to measure beam focusing properties, for
instance by arranging a beam profile sensor for the multitude of
beamlets downstream of the projection lens assembly during
construction, and measuring corresponding beam profiles while
adjusting the distance. In an alternative embodiment the distance
is adjusted to a predetermined value.
[0126] In summary the present invention discloses a projection lens
assembly for directing a multitude of charged particle beamlets
onto an image plane located in a downstream direction, and a method
for assembling such a projection lens assembly. In particular the
present invention discloses a modular projection lens assembly with
enhanced structural integrity and/or increased placement precision
of its most downstream electrode.
[0127] It is to be understood that the above description is
included to illustrate the operation of the preferred embodiments
and is not meant to limit the scope of the invention. From the
above discussion, many variations will be apparent to one skilled
in the art that would yet be encompassed by the spirit and scope of
the present invention. For instance, the principle of the invention
can also be applied to projection lens assemblies for directing one
or more beams of light onto an image plane. In this case the
electrodes may be replaced with light optics, and the beam blanker
may be replaced with a light modulator. As a further example,
multiple beamlets may pass through the same holes in the lens hole
array of the first and second electrodes. Moreover, a projection
lens assembly according to the invention may comprise any number of
electrodes greater than or equal to two without deviating from the
scope of the invention.
* * * * *